This invention relates to a method for refining glyceride oils by contacting the oils with adsorbents capable of removing certain impurities. More specifically, it has been found that glyceride oils can be treated with a combination of materials which serves to remove phospholipids, soaps and the like, facilitating decolorization of the oil by filtration through a packed bed of a pigment removal agent. This new process can be used in physical refining or in caustic refining operations. In the latter, it will be particularly useful in the presence of high soap levels, that is, even in the absence of the water wash centrifuge treatment typically required following caustic treatment. The disclosed method produces commercially acceptable oil products having substantially lowered concentrations of the indicated impurities.
For purposes of this specification, the term "impurities" refers to soaps, phospholipids and chlorophyll. Gums or other mucillagenous materials, if present, are also meant to be included. The phospholipids are associated with metal ions and together they will be referred to as "trace contaminants." The term "glyceride oils" as used herein is intended to encompass both vegetable and animal oils. The term is primarily intended to describe the so-called edible oils, i.e., oils derived from animal fats or from fruits or seeds of plants and used chiefly in foodstuffs, but it is understood that oils whose end use is as non-edibles (i.e., technical grade oils) are to be included as well. The invention is particularly applicable to oils which have been subjected to caustic treatment, which is the refining step in which soaps are formed in the oil. The invention also will find utility in physical refining, where the oil is not contaminated by soaps but where phospholipids are present and where residual gums may be present even following degumming steps.
Refining of crude glyceride oil purifies the oil of many undesirable substances, including gums, pigments (such as green (chlorophyll A), red (carotene) and yellow (xanthophyll) color bodies), phospholipids, free fatty acids and other volatile species that impart undesirable colors, flavors and odors to the oil. Removal of these species results in oil having good appearance, flavor, odor and stability. Many of these species are removed by contacting the oil with an adsorbent (i.e., bleaching earths or amorphous silica).
Crude glyceride oils, particularly vegetable oils, are refined by a multi-stage process, the first step of which typically is "degumming" by treatment with water or with a chemical such as phosphoric acid, citric acid or acetic anhydride. This treatment removes some but not all gums and certain other contaminants. Some of the phosphorus content of the oil is removed with the gums although significant levels of phospholipids still may be present. Either crude or degummed oil may be treated in either a physical or a chemical (caustic) refining process. The physical refining process includes a pretreating and bleaching step, and a steam refining and deodorizing step. No caustic refining step is used. Alternatively, the oil may be refined by a chemical process including neutralization (caustic treatment), bleaching and deodorization steps.
In chemical refining, the addition of an alkali solution, caustic soda for example, to a crude or degummed oil causes neutralization of free fatty acids to form soaps. This step in the refining process will be referred to herein as "caustic treatment" and oils treated in this manner will be referred to as "caustic treated oils." Soaps generated during caustic treatment are an impurity which must be removed from the oil because they have a detrimental effect on the flavor and stability of the finished oil. Moreover, the presence of soaps is harmful to the adsorbents used in vacuum bleaching and to the catalysts used in the oil hydrogenation process.
Current industrial practice is to first remove soaps by centrifugal separation (referred to as "primary centrifugation"). In this specification, oils which have been subjected to caustic treatment and primary centrifugation will be referred to as "partially refined" oil. Conventionally, the caustic refined oil, which still has significant soap content, is subjected to a water wash, which dissolves the soaps from the oil phase into the aqueous phase. The two phases are separated by centrifugation, although complete separation of the phases is not possible, even under the best of conditions. The light phase discharge is water-washed oil which now has reduced soap content. The heavy phase is a dilute soapy water solution. Frequently, the water wash and centrifugation steps must be repeated in order to reduce the soap content of the oil below about 50 ppm. The water-washed oil (or "refined" oil) is often dried to remove residual moisture to between about 2500 and about 1000 parts per million. The dried oil is then either transferred to the bleaching process or is shipped or stored as refined oil.
A significant part of the waste discharge from the caustic refining of vegetable oil results from the water wash centrifuge step used to remove soaps. In addition, in the caustic refining process, some oil is lost in the water wash process. Moreover, the dilute soapstock must be treated before disposal, typically with an inorganic acid such as sulfuric acid in a process termed acidulation. It can be seen that quite a number of separate unit operations make up the soap removal process, each of which results in some degree of oil loss. The removal and disposal of soaps and aqueous soapstock is one of the most considerable problems associated with the caustic refining of glyceride oils.
In addition, color bodies and phosphorus-containing trace contaminants must be removed from the oil. The presence of these trace contaminants can lend off colors, odors and flavors to the finished oil product. These compounds are phospholipids, with which are associated ionic forms of the metals calcium, magnesium, iron and copper. For purposes of this invention, references to the removal or adsorption of phospholipids is intended also to refer to removal or adsorption of the associated metal ions. In the removal of color bodies, attention is primarily given to the removal of chlorophyll.
Clays or bleaching earths commonly have been used for removing phospholipids and color bodies from glyceride oils by batch addition to the vacuum bleacher. These adsorbents may be used in their naturally occurring form or they may be acid-activated prior to use (U.S. Pat. No. 4,443,379 (Taylor et al.)). It is also known that amorphous silicas may be used in the oil refining process. U.S. Pat. No. 4,629,588 (Welsh et al.) teaches the utility of amorphous silica adsorbents for the removal of trace contaminants, specifically phospholipids and associated metal ions, from glyceride oils.
In current refinery practice, chlorophyll is most efficiently removed from glyceride oils by the use of acid-activated clays. Although commonly used in the industry, clays and bleaching earths suffer from a number of disadvantages. They typically do not filter well and require the addition of costly filter aids. Clays are associated with significant oil losses. Moreover, the presence of soaps and phospholipids in the oil is known to interfere with the clays' ability to remove chlorophyll. It is for this reason that one or more water wash centrifuge steps typically are required in caustic refined oil operations, in order to remove the soaps before the oil contacts the clay or bleaching earth.
Due to the presence of soaps (in chemically refined oil) or phospholipids (in physically refined oil), it has not previously been possible to use bleaching earths and clays in a packed bed format as taught by this invention. Conventionally, the bleaching material is added in a batch or slurry format and is subsequently filtered from the oil. It is known that chlorophyll removal capacity increases as the filter becomes coated with clay, thus forming a packed bed in situ through which the oil is filtered. The industry has attempted to take advantage of this packed bed (or "press bleach") effect by partially pre-coating th filter with a portion of the clay, perhaps up to about 20%, with the remainder being added to the vacuum bleacher in the usual (batch or continuous) fashion. This mixed addition format approach is the closest the industry has been able to get to utilizing a packed bed for decolorization of the oil. The mixed addition format initially yields filtered oil with a high chlorophyll content which drops over time as the packed bed builds up. However, due to the relatively short filter life achieved by this mixed approach, by the time chlorophyll removal capacity is maximized by the build-up of the bed, the filter must be changed.
Thus, although the advantages of a packed bed have been recognized in terms of chlorophyll removal, attempts to utilize a strictly packed bed operation in practice have been frustrated. Even where caustic treated oil is subjected to water wash centrifuge steps, too much residual soap and phospholipid remains to allow the use of a packed bed exclusively. A layer of slime quickly builds up on the oil/clay interface, causing severe pressure drop and preventing throughput of the oil. Residual gums and phospholipids present in physically refined oil cause similar sliming problems and pressure drop. The filter life is extremely short. Where no water wash centrifuge step is used, a packed bed would be completely nonfunctional. Prior art use of a packed bed in this process has therefore been limited to only partially pre-coating the filter, while still using continuous clay or bleaching earth addition in the vacuum bleacher (i.e., a semi-batch process).